The present invention relates to a color image forming apparatus, and more particularly to a color image forming apparatus of the type in which images formed by a plurality of image forming units are superimposed on an intermediate transfer belt, and those superimposed images are collectively transferred onto a recording sheet.
In a conventional color Xerography basis image forming apparatus, a called tandem type image forming apparatus is employed for the purpose of increasing printing speed. In this type of the apparatus, image forming sections or units are arranged side by side. Toner images are successively superimposed to form a color image.
Each image forming unit is formed with a photo-receptor unit for forming an electrostatic latent image and a developing unit for supplying toner to the photo-receptor unit.
In this type of the image forming apparatus, the image forming units are arranged side by side. Accordingly, the apparatus is long in the direction in which the image forming units are arranged. Further, in a case of where images are superimposed one on another on a recording paper, the fixing units are also arranged side by side. A length of the image forming apparatus is further increased. Consequently, the tandem type image forming apparatus needs an apparatus installing area.
A technique to solve the problem is disclosed in JP-A-11-15227, for example. The technique is directed to the image forming apparatus of the vertically mounting, tandem type.
FIG. 8 is a view schematically showing a conventional image forming apparatus. The conventional image forming apparatus is composed of a plurality of photo-receptors 28a (Bk), 28b (C), 28c (M) and 28d (Y) vertically arranged, image forming means for forming images of different colors on the photo-receptors 28a to 28d, a transport/transfer belt 37 for attracting a transfer sheet conveyed from a registration roller 12 which is disposed facing the photo-receptors 28a to 28d, and a fixing unit 13 for fixing a composite color image on a transfer sheet having been transported by the transport/transfer belt 37. In the image forming apparatus of the type in which images are superimposed from the photo-receptors 28a to 28d onto a transfer sheet having been transported by the transport/transfer belt 37, a length of the transfer-sheet transporting path ranging from the registration roller 12 to the first photo-receptor 28a (Bk) (=photoreceptor at which the transfer sheet first arrives) of those photo-receptors 28a to 28d is selected to be ½ or shorter than a length of the passable transfer sheet having the smallest size as viewed in the sheet passing direction.
Further, the publication discloses the image forming apparatus in which a length of the transfer-sheet transporting path ranging from the registration roller 12 to the attracting means is selected to be ½ or shorter than a length of the passable transfer sheet having the smallest size as viewed in the sheet passing direction.
Additionally, the publication discloses the image forming apparatus in which a length of the transfer-sheet transporting path ranging from the photo-receptor 28d (Y) (=photoreceptor at which the transfer sheet last arrives) of those photo-receptors 28a (Bk), 28b (C), 28c (M) and 28d (Y) to the fixing unit 13 is selected to be ½ or shorter than a length of the passable transfer sheet having the smallest size as viewed in the sheet passing direction.
In the conventional image forming apparatus of the vertically mounting, tandem type, the transport/transfer belt 37 electrostatically attracts a transfer sheet from a sheet supply cassette 9 located in a lower part of the apparatus, transports the transfer sheet in a vertical direction, and successively superimposes color images formed by image forming units 38a (Bk), 38b (C), 38c (M) and 38d (Y) on the recording medium to thereby form a color image. Following this, the transfer sheet bearing the thus formed toner image thereon is separated from the transport/transfer belt 37, and transported to a fixing device, and the toner image is fixed on the transfer sheet. A height of the image forming apparatus manufactured by the conventional technique described above is relatively high, but an area occupied by the apparatus when it is installed is reduced.
In the conventional technique, the toner images are transferred onto a recording medium, such a sheet of paper, in a superimposing manner. Accordingly, the thickness and a moisture absorbing property of the sheet (transfer sheet) and others affect the image transfer. This results in insufficient stability of the image reproduction.
Where a thick recording sheet is passed through the transporting path, a stiffness of the thick recording sheet is strong. Accordingly, the sheet transporting force of the registration roller or the fixing unit is larger than an electrostatic attracting force of the transport/transfer belt. As a result, the recording sheet tends to slip at the attracting part, possibly causing the out-of-color-registration
In the event of jamming such as paper jamming during a continuous printing operation, in particular in the event that the recording medium is left at a portion where the images are successively transferred on the recording medium, it is difficult to remove the jammed sheet. In this case, the user handles the sheet having much color toner not fixed yet sticking to there. Accordingly, there is much chance that his/her hands and clothes are frequently soiled, and further the color toner particles fly into the air to contaminate air.
Further, in design, the space between the adjacent color image forming units is reduced as much as possible in order to reduce the apparatus height. Therefore, the component parts are required to be reduced in size, thickness and diameter. This results in insufficient rigidity of the image forming units per se and the drive means. Where the insufficient rigidity is present, it is difficult to secure the positioning accuracy of the image forming units and the drive accuracy as major factors in the image formation. Accordingly, the color image formed is deteriorated in the out-of-color-registration and the color irregularity of the color image.
Further, the image forming units per se are reduced in size. Accordingly, a space used by the user for replacement of the image forming units as expendables is narrow, and the operability is deteriorated in the replacement work.